Mottle can be generally defined as the irregularities in the visual appearance of a sheet of paper or paper-like material. The irregularities can be caused by variations in the color, color intensity, or relative amount of gloss. Mottled appearance is also caused by variations in the coatings applied to the surface of the paper. Certain types of coated paper, such as paper having a coating of filled polyethylene etc., often have a mottled appearance if there is a "show through" of the underlying irregularities through the coating. Such surfaces may still exhibit high gloss and a smooth texture. A similar, but slightly different, variation of mottle is obtained when a white blocking coating is applied over the surface of a paper.
Currently, the degree of mottle is determined by highly subjective, visual inspection. Subjective ratings of mottle can vary widely between operators and between customers and their manufacturers. There has been a long felt need in the paper industry to obtain an objective standard by which to measure mottle which is related directly to the established subjective standards. There has also been a long felt need to evaluate the degree of mottle on a consistent objective basis.
Various prior methods have been employed to determine the surface properties of paper and other materials. Some have included radiation sources and collectors capable of receiving a portion of the radiation being reflected from a sheet of material and detectors capable of measuring select properties of the sheet based upon the radiation received. See U.S. Pat. Nos. 4,277,177, 4,319,847. Others have employed elaborate lens systems or multiple photodetectors for determining roughness or surface defects. See U.S. Pat. Nos. 4,364,663 and 4,465,371. Additionally, the use of tuned amplifiers and Fourier transform spectrums which rely on the differentiation in diffraction patterns have been employed for increasing the sensitivity of detection of flaws in a surface. See U.S. Pat. Nos. 4,465,371 and 4,338,822. Despite these teachings, however, the paper-making art has not generated a suitable apparatus for making an objective determination of mottle.